The present invention relates to electromagnetic actuators, and, more particularly, to a circuit for controlling and driving a single winding electromagnetic actuator, commonly referred to as a Voice Coil Motor (VCM).
VCM actuators are used in numerous applications, such as, for example, hard disk drive (HDD) systems to position the read/write heads on the different tracks of the disk. One of the operations carried out in a HDD system by a VCM actuator is the positioning of the read/write head on a ramp for preventing impacts of the head on the surface of the disk when the equipment is subjected to shocks or abrupt movements.
These operations are commonly called ramp load and ramp unload operations. It is normally necessary to control the speed of the mechanical arm that supports the head during its swinging movements for carrying out these operations. The methods used to monitor the instantaneous speed of the arm include reading the back electromotive force (BEMF) whose level is proportional to the speed of the motor.
Fundamentally, this reading may be made using a circuit that reads the voltage during a null current condition in the winding of the motor. Under these conditions, the voltage detected corresponds only to the BEMF contribution given by the general equation:   E  =            L      *                        ⅆ          i                          ⅆ          t                      +          R      *      i        +    BEMF  
The null current condition is usually obtained by momentarily placing in a condition of high impedance (tristate) the power stages that drive the winding of the motor. The duration of the high impedance is sufficient for delaying to zero the discharge current of the inductance of the winding through a recirculation path.
Known drive and control systems use a dedicated operational amplifier (OP-AMP) for detecting the BEMF. Moreover, such systems commonly include a second operational amplifier in the transconductance regulation loop circuit for driving the VCM that is used to monitor the current in the motor.
The BEMF is read after having opened the transconductance regulation loop and upon the discharge of the current in the winding of the VCM. Therefore, the BEMF voltage is read during an operating phase in which the information on the current in the motor is not required.
It is an object of the present invention to provide a control driving system based on the use of a unique operational amplifier. The operational amplifier is in the regulation loop for driving the electromagnetic actuator, and for measuring during distinct phases of operation the current of the motor and the induced BEMF.
This approach allows for a reduction of the area of integration of the system as a whole. The control and driving system, according to a first embodiment, comprises a single operational amplifier having two differential input stages in parallel with each other.
The first differential input stage has a noninverting input coupled to a reference voltage, and an inverting input connected to the output of the operational amplifier to form a transconductance feedback structure. The second differential input stage has the noninverting and inverting inputs respectively switchable to two nodes coincident with the terminals of the current sensing resistance connected in series to the winding of the electromechanical actuator. Alternatively, the noninverting and inverting inputs are respectively switchable to the two output nodes of the pair of power stages that drive the winding of the electromechanical actuator in a bridge configuration during a phase of detection of the electromotive force induced in the winding.
According to an alternative embodiment functionally equalivant to the first embodiment, the operational amplifier is equipped with three distinct differential input stages. The first differential input stage forms the feedback circuit, and may be similar to the one used in the embodiment previously described. The respective inputs of the second and third differential input stages are coordinately coupled to the two nodes coincident with the terminals of the current sensing resistor. The first differential input stage is coupled to the output nodes of the two power stages that drive the winding. A pair of path selectors select one of the second and third differential input stages for connecting in parallel to the first differential input stage.